Cryptonomicon is a 1999 novel by American author Neal Stephenson, drawing thematic parallels between wartime secrecy and modern information control, emphasizing the role of information in political, military, and personal domains.

The story is set in two different time periods. One group of characters are World War II–era Allied codebreakers and tactical-deception operatives affiliated with the British Government Code and Cypher School at Bletchley Park, and disillusioned Axis military and intelligence figures. The second narrative is set in the late 1990s, with characters that are (in part) descendants of those of the earlier time period, who employ cryptologic, telecom, and computer technology to build an underground data haven in the fictional Sultanate of Kinakuta. Their goal is to facilitate anonymous Internet banking using electronic money and (later) digital gold currency, with a long-term objective to distribute Holocaust Education and Avoidance Pod (HEAP) media for instructing genocide-target populations on defensive warfare.

Cryptonomicon is closer to the genres of historical fiction and contemporary techno-thriller than to the science fiction of Stephenson's two previous novels, Snow Crash and The Diamond Age. It features fictionalized characterizations of such historical figures as Alan Turing, Albert Einstein, Douglas MacArthur, Winston Churchill, Isoroku Yamamoto, Karl Dönitz, Hermann Göring, and Ronald Reagan, as well as some highly technical and detailed descriptions of modern cryptography and information security, with discussions of prime numbers, modular arithmetic, and Van Eck phreaking.

According to Stephenson, the title is a play on Necronomicon, the title of a book mentioned in the stories of horror writer H. P. Lovecraft:

I wanted to give it a title a 17th-century book by a scholar would be likely to have. And that's how I came up with Cryptonomicon. I've heard the word Necronomicon bounced around. I haven't actually read the Lovecraft books, but clearly it's formed by analogy to that.

The novel's Cryptonomicon, described as a "cryptographer's bible", is a fictional book summarizing America's knowledge of cryptography and cryptanalysis. Begun by John Wilkins (the Cryptonomicon is mentioned in Quicksilver) and amended over time by William Friedman, Lawrence Waterhouse, and others, the Cryptonomicon is described by Katherine Hayles as "a kind of Kabala created by a Brotherhood of Code that stretches across centuries. To know its contents is to qualify as a Morlock among the Eloi, and the elite among the elite are those gifted enough actually to contribute to it."

The action takes place in two periods—World War II and the late 1990s, during the Internet boom and the Asian financial crisis.

In 1942, Lawrence Pritchard Waterhouse, a young United States Navy code breaker and mathematical savant, is assigned to the newly formed joint British and American Detachment 2702. This ultra-secret unit's role is to hide the fact that Allied intelligence has cracked the German Enigma code. The detachment stages events, often behind enemy lines, that provide alternative explanations for the Allied intelligence successes. United States Marine sergeant Bobby Shaftoe, a veteran of China and Guadalcanal, serves in unit 2702, carrying out Waterhouse's plans. At the same time, Japanese soldiers, including mining engineer Goto Dengo, a "friendly enemy" of Shaftoe's, are assigned to build a mysterious bunker in the mountains in the Philippines as part of what turns out to be a literal suicide mission.